Conventionally, an update of map data in a navigation system equipped with a rewritable map data storing means is performed by temporarily detaching the map data storing means from the navigation system and then updating the map data using a special-purpose map data rewriting means, and attaching the map data storing means again to the navigation system, or by stopping the navigation system to copy update map data to the map data storing means using an update map data acquiring means by means of a system loader (for example, refer to patent references 1 to 3).    Patent reference 1: JP,2001-165675,A    Patent reference 2: JP,2001-165669,A    Patent reference 3: JP,2003-315052,A
A problem with an update of map data in a prior art navigation system is, however, that because it is performed by temporarily detaching a map data storing means for storing the map data from the navigation system and then attaching the map data storing means again to the navigation system, or by stopping the navigation system to copy update map data to the map data storing means, the functions of the navigation system cannot be used at all during the update of the map data.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a navigation system which, when updating map data, enables the update of the map data while using a navigation function by starting writing update map data in a free space on a map data storing means.